Spontaneous hypertension is studied in a strain of rats which is highly prone to show severe hypertension with hypertensive pathological lesions. By using a combination of fluorescent tracers, intravital microscopy and electron microscopy, the development and progression of arterial and arteriolar abnormal endothelial permeability, sustained smooth muscle contraction and relaxation, intimal amd medial hyperplasia, fibrinoid deposition, necrosis and fibroplasia will be studied. The effect of vasoactive agents on the abnormally contracted and/or dilated arteries and arterioles is tested, applied locally by microiontophoresis. A continuous recording of the blood vessels on video tape is made during the fixation and embedding procedures. The data thus recorded can be played back instantly at any time for evaluation, measurement, identification, and control of the precise initial sites, within the vascular tree, of abnormal arterial and arteriolar reactivity. BIBLIOGRAPHIC REFERENCES: Rhodin, J. Microscopic anatomy of the pulmonary microcirculatory bed in the cat lung. Microvascular Res. (1977). In press. Jokelainanen, P.T., Haack, D.W., Simpson, J.G. and Rhodin, J.A.G. Deposition of silver in vivo in basal laminae of different capillary types. Anat. Rec. (1977). In press. Abstract.